This invention relates to processes of electrochemical milling. More particularly, the invention addressed itself to electrochemical milling where the material being removed covers an electrically nonconductive material which cannot withstand elevated temperatures, such as an explosive or rocket propellant.
The process of electrochemical milling and the use of maskants in the application of such a process to prevent material removal and thereby prescribe the shape of the etch are all known in the prior art; see U.S. Pat. Nos. 1,376,365 and 3,380,863 and the subject Electrochemical Metal Removal in VAN NOSTRAND'S SCIENTIFIC ENCYCLOPEDIA copyright 1968. This process has been used to dissect solid propellant rocket motors, or other metal encased highly flammable materials, to allow analysis of the interior materials or structure. In the specific case which led to the invention disclosed herein, rocket motors were being periodically sampled and dissected to evaluate the effects of storage time and environment upon the propellant composition. Electrochemical milling was chosen because it is generally considered relatively safe and is less expensive than other metal removal techniques such as grinding.
The etching was generally performed by first completely covering the rocket motor case with a maskant, a moisture proof tape, and then removing adjacent circumferential or axial strips of the tape in succession, allowing the exposed metal beneath each strip to be completely etched away before the next strip was removed. Depending on the nature of the motor, such removal and etching would continue until either the whole case was etched away or just sufficiently to allow the case to be separated and propellant removed.
Although not to the extent encountered with the other metal removal techniques, the use of electrochemical milling has produced a significant number of accidental propellant ignitions, resulting in the destruction of the dissection equipment and risk of injury to facility personnel. Analysis of the ignition mechanisms and recognition of causes was not straight forward, in the normal sense, since the accidental ignitions followed no prescribed pattern, reasonable modifications in the electrochemical processes did not prevent ignitions and, foremost, once ignition occurred the reaction was so violent that very little of the hardware remained for analysis.
The forementioned attempts at modification involved reductions in the area exposed to the etching electrolyte solution at any one time, careful monitoring of the etching process, and changes in the electrical potential being applied. None proved to be entirely successful and accidental ignition would sometimes occur.